Vibrational modes of trigonal Se and Te are analyzed to determine the character of the interchain (secondary) forces. The dispersion of low-frequency modes clearly demonstrates the directional (covalent) character of these forces. The interference of secondary interactions with the intrachain (primary) bonding is evidenced by the anomalous softening of the zone-center A₁ mode relative to other modes and to corresponding modes in other molecular forms of these elements. The nature of and trends in the forces are consistent with electronic calculations and may be interpreted as aspects of a metal-insulator transition.
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Martin et al. (1976) studied this question.